In a conventional method, stacked unit cells are fabricated by mixing a binder and additives by a wet process to prepare a slurry, subjecting the slurry to tape casting to prepare an electrolyte sheet, and stacking the electrolyte sheet between an air electrode, a fuel electrode and a matrix to supply the electrolyte to each cell.
However, the use of this method has disadvantages in that, because the electrolyte sheet is melted during pretreatment of the fuel cell stack so as to be absorbed into the pores of the air electrode, the fuel electrode and the matrix, the total height of the stack is reduced by the height of the melted electrolyte sheet to reduce the mechanical stability of the stack, and non-uniform melting of the electrolyte which occurs during the pretreatment process makes the surface pressure distribution of the stack non-uniform, thus further increasing the mechanical instability of the stack.
In the above conventional method, because the electrolyte thermally expands at a rate greater than the matrix during pretreatment and melts at the melting temperature to permeate into the matrix, it can cause the thin plate shaped matrix having low strength to crack. The organic material in the matrix sheet disappears due to decomposition during the pretreatment process, but the matrix sheet has no chemical bond between the sintered particles, and thus has low strength. Thus, the matrix sheet is more likely to crack than the electrode, and for this reason, gas crossover between the fuel electrode and the air electrode will occur, thus adversely affecting the performance and lifetime of the fuel cell stack.
In attempts to overcome the above-described disadvantages, Korean Patent Application Nos. 10-1999-0046201, 10-2005-0020973, 10-2006-0112314 and 10-2006-0132459 disclose a method of using ceramic fiber as a reinforcing material to increase the thermal stability of the matrix, or using a sintering aid so as to be capable of increasing the bonding strength between the matrix particles at the operating temperature of the molten carbonate fuel cell, or adding a porous metal support.
However, in the method of using the ceramic fiber, agglomeration of the fiber occurs during the slurry preparation process, or the fiber is oriented in the direction of movement of the matrix sheet, so that defects occur during production of the matrix, thus reducing the production yield. Also, the matrix provided according to this method has low resistance against the differential pressure between the air electrode and the fuel electrode, indicating that this method does not yield the expected effect. Meanwhile, in the method of using the sintering aid, the bonding strength between the matrix particles shows a tendency to increase, but the fine pores of the matrix are changed to reduce the capillary force, so that the matrix cannot retain an electrolyte. Meanwhile, in the method of using the porous metal support, the high surface pressure that is applied during the stacking process causes high stress around the support, indicating the support shows an adverse effect during pretreatment. As a result, although the above-described methods aim to increase the mechanical strength of the matrix, these methods do not present a complete solution.